Never use the name. Names are trademarked and thus you should stay clear of them. You can modify the name but be sure that it's not the same.

Avoid exact textures and models. Even if you create art yourself, if it's something that the CS people consider theirs, you may be in trouble.

No problem on the HUD or gameplay. These elements can't be copyrighted or patented or trademarked. Hence you can replicate that style of HUD and play--but only the style--safely.

In short, stay away from copying anything art related: textures, models, levels, music. Instead, distill the essense of CS and make everything else yourself.